In various manufacturing processes articles are prepared that may include isolated areas of contrast variations on their exposed surfaces. These isolated contrast variations may be attributable to inhomogeneities within the materials. That is, the surfaces may contain spots or streaks on a background of essentially uniform color and/or hue. In many cases, these contrast variations have no deleterious effect upon the use articles for an intended purpose. However, if a surface contains a sufficient number of contrast variations or the respective quantities and areas of the variations exceed predetermined standards, the articles may not be considered acceptable in some commercial channels. In order to meet a particular industry's cosmetic or performance standards, it is necessary to inspect each article for contrast variations before it is shipped to a consumer. The articles may be visually inspected by a properly trained human. Generally, a human inspector requires substantial training and the inspection results are always subjective. Therefore it is desirable to employ an automated apparatus and method that performs an objective inspection of surfaces for contrast variations.
Surface inspection methods and apparatus are particularly useful in the semiconductor industry. There, some automated apparatus and methods for inspecting polished surfaces of semiconductor wafers, such as silicon and gallium arsenide, are known. Generally, those surfaces are searched for three dimensional features, such as scratches, etch pits and foreign particles, i.e., defects, rather than for two dimensional contrast variations. That apparatus is generally expensive and usually employs a scanning laser light source or an ultraviolet light source. The known apparatus exploits the mirror-like properties of the polished surface in searching for defects. Therefore, this apparatus is not able to search an unpolished surface for defects or to search any type of surface for contrast variations.
The surface of many articles useful in industrial processes are never polished. Therefore, the known automated inspection apparatus and methods cannot be used to locate defects or contrast variations on the surface of those articles. An example of articles used in the semiconductor industry that are not polished, but which must be inspected, are wafers that are employed as dopant sources for doping silicon semiconductor wafers. However, the invention and its application is not limited or restricted to articles of a particular shape, composed of any particular element, compound of mixture or intended for a particular use.
Accordingly, it is an objective of the invention to devise an apparatus and to achieve an automated method for objectively inspecting a surface of an article for isolated contrast variations. It is a further objective of the invention that the inspection method permit acceptance or rejection of inspected surfaces according to established criteria concerning the number of contrast variations and their respective areas that may acceptably appear on an exposed surface.